Recent advancement of information processing systems using the electrophotographic method is remarkable. In particular, laser printers and digital copiers which convert information to digital signals and record the information by using light have been remarkably improved in terms of their printing quality and reliability. Moreover, by being integrated with speed-enhancement technologies, these have been applied to laser printers and digital copiers capable of full-color printing.
As photoreceptors used for these laser printers, digital copiers, and the like employing the electrophotographic method, photoreceptors using organic photoreceptor materials (OPCs: organic photoconductors) have been generally and widely applied because of their costs, productivity, pollution free characteristics, and the like.
Recently, as the diameters of the photoreceptors get smaller (have shorter times taken from exposure to development) with the speedup or size-reduction of electrophotographic devices, high-speed response of charge transport materials for electrophotographic photoreceptors becomes increasingly important.
As the charge transport materials, tetraphenylbutadiene derivatives, hydrazone derivatives, triphenylamine derivatives, stilbene derivatives, and the like are used. Moreover, 4,4-diphenyl-1,3-butadienyl group-containing triphenylamine derivatives having longer conjugated systems (for example, 4,4′,4″-tris(4′″,4′″-diphenyl-1′″,3′″-butadienyl)triphenylamine (Japanese Patent Application Publication No. Hei 8-295655)) than stilbene derivatives and the like exhibit high charge transport ability. Many patent applications have been filed also for butadienyl derivative (Japanese Patent Application Publication No. Hei 9-34142, Japanese Patent Application Publication No. 2004-252066, Japanese Patent Application Publication No. 2005-289877, and Japanese Patent Application Publication No. 2008-63230).
In addition, 4,4′,4″-tris(4′″-(4″″-phenyl-1″″,3″″-butadienyl)styryl)triphenylamine), which is one of the organic compounds having longer conjugated systems, is reported to be used as a light-emitting dendrimer (International Application Japanese-Phase Publication No. 2003-522202 and Japanese Patent Application Publication Nu. 2012-156519).
In general, a charge transport layer is a solid solution film in which molecules of any of these low-molecular weight charge transport materials are dispersed in a binder resin and which is approximately 10 to 30 μm in thickness. In addition, for most electrophotographic photoreceptors, a bisphenol-based polycarbonate resin, a polyarylate resin, or a copolymer of any of these resins with another resin is used as the binder resin.
For forming the charge transport layer, the binder resin and the low-molecular weight charge transport material are dissolved in an organic solvent, and a film is formed therefrom. However, it cannot be said that conventional low-molecular weight charge transport materials are sufficiently soluble in the binder resin and the organic solvent. Moreover, although some of the low-molecular weight charge transport materials are soluble and films can be formed therefrom, the carrier mobilities of charge transport layers using the conventional low-molecular weight charge transport materials are not sufficiently high.
Hence, it cannot be said that an electrophotographic photoreceptor can be obtained which has excellent electrophotographic photoreceptor characteristics such as high sensitivity and low residual potential and moreover whose charge transport layer can be formed by a simple formation step and is in a stable state.